In a cellular system, handover is performed along with the movement of a mobile station to switch cells (base stations) to which the mobile station belongs. The handover is performed based on channel quality of neighboring cells measured by the mobile station so that the mobile station is handed over to an appropriate cell. Meanwhile, there is a case where frequency carriers used in the neighboring cells (including the current serving cell) are different from the frequency carrier currently being used by the mobile station for communication (such frequencies are hereafter called different frequencies), or multiple frequency carriers are used in each neighboring cell. Also, there is a case where cells using different wireless access methods (different systems or alternative systems) are present around the mobile station and it is better, in terms of the traffic volume and propagation conditions, to hand over the mobile station to one of such cells to smoothly continue the communications. In such cases, a mobile station is preferably configured to be able to measure channel quality of different frequencies and systems (i.e., to perform a cell search in a system with a different frequency) during communication.
Here, it must be noted that a mobile station with only one receiver cannot tune to multiple frequencies and systems at the same time. This is because the radio frequency (RF) circuit of a receiver cannot tune to multiple frequency carriers and systems at the same time. To be able to measure the channel quality of multiple frequency carriers and systems at the same time, a mobile station must be equipped with multiple receivers (RF circuits). However, this increases sizes, prices, and power consumption of mobile stations. For this reason, many mobile stations currently being used are equipped with one receiver. Such a mobile station measures channel quality of different frequencies and systems during interruptions (may also be called a gap or a gap period) generated by discontinuous reception (DRX) where the current communication is stopped intermittently. In other words, the mobile station stops tuning to a frequency, tunes to a different frequency, and thereby performs a cell search with the different frequency. In this case, if the base station does not know the gaps in DRX, the base station may send signals during the gaps when the mobile station cannot receive signals of the current serving system. This results in waste of precious radio resources as well as causing disadvantageous effects such as an increase in interference power to other communications and an increase in transmission delay. Therefore, the base station has to correctly recognize the DRX status of the mobile station.
In conventional communication systems, DRX is controlled using wireless protocols without taking into account radio channel quality. For example, in W-CDMA, periods (gaps) for measuring the channel quality of different frequencies and systems are provided by starting a compressed mode using a radio resource control (RRC) protocol. Generally, in a compressed mode, a gap is created by transmitting data, which are normally sent using one frame, within a half of the time by doubling the transmission rate. However, in the compressed mode, gaps are created according to a predetermined pattern and therefore the timings of the gaps are determined without taking into account the radio channel quality. Therefore, even if the radio channel quality is good enough to efficiently transmit data, the transmission of data is prohibited if it coincides with the timing of a gap. As a result, the unsent data may have to be transmitted at a timing when the radio channel quality is poor. Thus, with conventional methods, the transmission efficiency may be reduced. The RRC protocol is described, for example, in 3GPP TS25.331 “Radio Resource Control (RRC) Protocol Specification”, v6.8.0.